This invention relates to an oxygen indicator adapted for printing and also relates to a sheet-like oxygen indicator.
The color of an oxygen indicator changes from one color in an aerobic atmosphere to a different color in an anaerobic atmosphere. So, whether oxygen is present or not in the atmosphere in which the indicator is placed is easily determined by its color.
An oxygen indicator comprising an aqueous solution of sodium hydroxide, glucose and Methylene Blue has been known.
Recently, oxygen-free packaging of foodstuff, e.g. vacuum-packed package, package containing an oxygen absorbent, nitrogen filled package, and so on, have been widely demanded in order to prolong the shelf-life of the foodstuff. A convenient oxygen indicator which has high sensitivity and long life is necessary for the control of such packing systems. However, the shelf-life of the prior art oxygen indicators is too short for such a purpose.
Some of the present inventors have proposed an oxygen indicator in a powder state or a tablet state. Such oxygen indicator is disclosed in U.S. Pat. No. 4,169,811 by Yoshio Yoshikawa et al. patented on Oct. 2, 1979 which is incorporated herein as reference.